In closed loop hydrostatic systems, loss of hydraulic fluid occurs due to leakage at movable parts or due to loop flushing, for instance, for cooling high loaded parts or the hydraulic fluid itself. For example, hydraulic fluid flowing through a closed loop hydraulic circuit can escape through internal leaks in the pump and/or the motor, thereby decreasing the system pressure below acceptable margins. In addition, because the hydraulic circuit is closed, fluid circulating in the closed loop can overheat under load conditions. To compensate for the escaping and overheated fluid, closed loop circuits frequently employ fixed displacement pumps, also known as charge pumps having a lower conveying volume than the main pump. These charge pumps are driven commonly by a driving shaft of the main pump or any other drive engine with a constant transmission ratio, i.e. the rotational speed of the charge pump changes proportionally with the rotational speed of the main pump. This means also, that at low rotational speeds of the main pump, the charge pump flow rate is also on a low level and rises proportionally with the increase of the rotational speed of the main pump. As the charge pump capacity has to be sufficient to maintain the hydraulic fluid pressure in the closed hydrostatic circuit also at low rotational speed of the main pump, the charge pump flow rate generated at high speed frequently exceeds the hydraulic fluid losses due to the fix charge pump capacity. Therefore, in these conditions the charge pump flow rate has to be limited by a charge pump pressure relief valve. Usually, the exceeding charge pressure flow rate is released to a tank, normally to the same tank, from which the charge pump sucks hydraulic fluid to fill up the closed hydrostatic circuit.
To avoid these parasitic losses or to avoid surcharging the closed hydrostatic circuit, different solutions in the state of the art have been proposed which cannot eliminate completely these parasitic losses. US 1977H1 (EPO: USH1977H1) provides with a fixed displacement charge pump whose charge flow rate is controlled by an electrohydraulic proportional relief valve controlled by the main controller of the hydrostatic transmission. This system does not use the system pressure for limiting the charge pump flow rate. An excess of charge pump flow rate is still generated at high rotational speeds of the main pump. According to US 1977H1 such an excess of charge pump flow rate is conducted to a tank.
US 2008/0238187 A1 proposes a variable displacement pump for providing a variable charge flow. Thereby, the displacement of the variable charge pump is controlled by a pressure sensor and an actuation device. Here, parasitic losses due to an excess of generated charge fluid flow rate is avoided. By means of the variable displacement pump the charge flow rate can be reduced. Nevertheless, the variable charge pump has to be capable to provide enough charge flow rate to the closed hydrostatic circuit at low rotational speeds of the main pump of the closed hydrostatic circuit. Thus, the capacity of the charge pump at low rotational speeds must be relatively high, i.e. the maximum volumetric size of the variable charge pump has to be relatively high, as the conveying volume of the charge pump is determined at maximum displacement and minimum rotational speed. If the rotational speed is rising, the displacement of the charge pump can be reduced in order to avoid an excess in hydraulic fluid supply. Thus, during operation of the main pump at middle or high revolution speeds the variable charge pump will always be at low displacement. Thus, most of the time a relative high volumetric variable charge pump has to be driven at low displacement. This constitutes parasitic losses, as more power than necessary is applied.